1. Field of the Invention
The invention relates to a circuit for driving a flat panel display in a sub field mode. Information to be displayed is provided as a succession of frames, each to be displayed during a corresponding field period. Such a display includes a plurality of display elements arranged in a matrix of rows and columns, and a plurality of first electrodes, each first electrode of the plurality of first electrodes being associated with display elements in a respective row or column. A circuit of this type includes a timing generator which divides a field period of a received display information into consecutive sub field periods, each sub field period including an address period preceding a display period, and each sub field period having a respective weight factor associated therewith. A drive circuit supplies drive signals, during the sub field periods corresponding to the respective weight factors, to respective addressed electrodes of the plurality of first electrodes. Each display element which is to be lit during a field period is addressed in one or more of the sub field periods, the sum of the weight factors associated with those sub field periods determining the luminance with which the display element is lit.
The invention also relates to a flat panel display apparatus having such a flat panel display and such a circuit for driving the flat panel display, and to a method of driving a flat panel display.
2. Description of Related Art
U.S. Pat. No. 5,541,618 discloses a method and a circuit for gradationally driving a flat panel device such as a Plasma Display Panel (further referred to as PDP). A PDP comprises a plurality of cells formed at cross points of scan electrodes and data electrodes which are arranged orthogonal to the scan electrodes. A picture to be displayed has a frame rate of 60 Hz. Each frame of the picture to be displayed is associated with a field period which is divided into a plurality of sub field periods. Each such field period comprises an address period and a display period. In each address period, the cells to be lit during the subsequent display period are addressed by sequentially selecting the scan electrodes and supplying appropriate data to the data electrodes for each selected scan electrode. In this way a desired charge is stored in the cells to be lit. During each display period sustain pulses are supplied to all the cells to light the cells in which the desired charge is stored. The brightness of a lit cell is determined by the number or the frequency of the sustain pulses. In a preferred embodiment, each display period has a different number of sustain pulses, and the frequency of the sustain pulses is equal for every display period. The number of sustain pulses of the display periods essentially have a ratio of 1:2:4:8: . . . 128. Therefore, the durations of the display periods have also this ratio. The cells or picture elements for the picture to be displayed are each represented by a binary coded data word in which each bit corresponds to one of the sub frames such that the length of the display period of that sub field is in accordance with the weight of the data bit in the data word. The cell is lit during the display period of a certain sub field of the bit of the data word associated with this certain sub field indicates such. So, the bits of the data word determine during which sub frames of a frame the cell produces light. The visual brightness of each cell is determined by the number of sustain pulses accumulated during the entire frame period.
It is a drawback of the method and the circuit for gradationally driving a PDP according to U.S. Pat. No. 5,541,618 that a large area flicker occurs in certain conditions. The large area flicker occurs most noticeably if large areas of cells are lit only during the sub field with the longest display period. A large contribution to the luminance output is generated during a very limited period in time during a frame. These light pulses occur with the repetition frequency of the frame. At a frame repetition frequency of 60 Hz, the eye might integrate the separate light pulses such that a flicker is not very annoying. But, at a frame repetition frequency of 50 Hz the gap in time between the light pulses is so large that the eye clearly detects an annoying flicker. A same reasoning holds if a large area of cells is lit during a sub field with a display period which is not the longest. However, the flicker will be somewhat less as the amount of flicker detected by the eye also depends on the amount of light generated.
It is an object of the invention to provide a drive for a flat panel display such that less flicker occurs.
To this end, a first aspect of the invention provides a circuit for driving a flat panel display having a plurality of first electrodes partitioned into two groups. The first group are driven by signal corresponding to sub field periods in which each of the sub field periods has a weight factor associated with it, and the weight factors for the corresponding sub field periods occur in a predetermined order. The second group is similarly driven by signals corresponding to sub field periods each having a respective weight factor, but the weight factors occur in a different predetermined order. A second aspect of the invention provides a flat panel display apparatus with a flat panel display and a circuit for driving the flat panel display as just described. A third aspect of the invention provides a method of driving such a flat panel display. Advantageous embodiments of the invention are defined in the dependent claims.
An AC plasma display is a bilevel display with a memory function, i.e. it can only turn pixels on or off. To switch a pixel on, a prime sequence (addressing period) is necessary. In such a sequence a pixel that should turn on is conditioned, in such a way, that it turns on when a voltage is put across the scan and sustain electrodes (during the display period). This is done for all pixels in a display that should turn on. The grayscale itself is now generated in such a way that the luminance value is divided into several subfields with various weights. When for a subfield in a display all pixels that should be turned on are primed, the scan and sustain voltage is put on the display for the sustain period corresponding to the weight of that subfield and all primed pixels turn on. In the next subfield this process is repeated for that subfield with the corresponding subfield weight. The weight of a subfield determines how long the pixels are turned on. The luminance value of a pixel is determined by the input byte of Red. Green or Blue (RGB). When the weight of the subfields correspond to the weight of the input bits of a pixel, the weight of a bit corresponding to the subfield weight determines whether this pixel is primed, i.e. whether this pixel is turned on during the sustain period.
When large areas are lit using only one subfield with a high bit weight, in only one moment in time a large contribution to the luminance output is generated. This results in large area flicker with large frequency components of 50 or 60 Hz for which the eye is quite sensitive. The method to be proposed reduces large area flicker behavior when planes of one grayscale are shown, especially when only a few subfields (MSB) generate the luminance in a filed. To overcome the large area flicker in these cases, the odd and even rows are addressed in different groups, and the subfield order of the odd and even rows are chosen differently from each other, so that the odd and even rows are in anti-phase with respect of each other for the subfields with the highest bit weights. This reduces the large area flicker considerable for the frequency components of 50 and 60 Hz. The eye observes both rows at the same time and will mainly see frequency components around 100 or 120 Hz for which the eye is less sensitive.
The above described sub field order is a preferred embodiment of the invention. It is also possible to reduce the flicker if groups of two or more consecutive rows each with a same first sub field order alternate with groups of two or more consecutive rows each with a same second sub field order. It is also possible to repeat a group of rows each having a different sub field proper. For example, a group of four successive rows is repeated, each of the rows out of the group of four has a different sub field order.
Although an optimal reduction of the flicker is obtained if the position of the sub fields with the highest bit weights is selected to be an anti-phase, any different position of these sub fields reduces the flicker.
JP-A-07271325 discloses a circuit for selecting different sub field orders during successive fields. Due to the different position of the sub fields in subsequent fields, the distance in time between corresponding sub fields varies, thereby deteriorating the flicker reduction. As in one field the same sub field order is supplied to every scan electrode, the scan electrodes need not be connected in groups, and the data bit order need not be changed within a field.
The publication 19.4 xe2x80x9cImprovement in PDP Picture quality by Three-Dimensional Scattering of Dynamic False contoursxe2x80x9d, by T. Yamaguchi et. al., SID 96 DIGEST, pages 291-294 discloses that disturbances of the grey level called dynamic false contours are reduced by splitting the two most significant sub fields into four sub fields with an equal length D. The length of the remaining sub fields is denoted by A. This publication does not refer to flicker improvement. The splitting of two sub fields into four sub fields has the disadvantage that two more address periods are needed. So, this method requires a 1.25 times faster switching operation of the discharge cells, or in practice, the total time for generating light and thus the light output decreases with 25%. This SID publication is concerned with signal processing which decodes the binary coded data words into drive signals which randomly select the right sub field in successive frames to obtain the light output corresponding to the data word. In this way, the light pulse occurrence is randomised in moment of occurrence. It is disclosed that a certain data value can be generated by the sub fields belonging to A, or by selecting one of the four sub fields D. It is further disclosed that from horizontal line to line in subsequent fields the order of the four sub fields D and the sub field A may be changed. There is no disclosure of any hardware measure enabling a different sub field order for different lines within one and the same field.
So, this publication discloses that the visibility of dynamic false contours is minimized by selecting a different sub field order in successive field periods. This is not an effective measure to reduce flicker. According to the invention, different sub field orders are applied to different groups of rows in a same field period thereby reducing the flicker. It has to be noticed that it is additionally possible to select different sub field orders in successive field periods for one or each of the groups of rows. In this way, the invention provides a solution to decrease the amount of flicker as well the visibility of the false contours.
In one embodiment of the invention, a display element is formed by the crossing of a scan electrode and a data electrode. In the prior art, this type of display is referred to as opposed-discharge type.
In another embodiment of the invention, a plasma panel sub-pixel (also referred to as cell or display element) is formed by the crossing of two row electrodes and a column electrode. The two row electrodes extend in the row direction. They are referred to as scan electrodes and sustain electrodes. Plasma channels may be aligned with the row or the column electrodes. Plasma cells may be used instead of plasma channels. In the prior art, this type of display is referred to as surface-discharge type.
In a preferred embodiment of the invention, the flicker is reduced by a large amount because the different sub field orders occur in consecutive lines and thus are optionally integrated by the eye.
In an embodiment according to another preferred aspect of the invention, the flicker is reduced by a large amount because sub fields which have a same bit weight are applied to one of the groups of the scan electrodes shifted over about a half field period in time with respect to the other group of scan electrodes. As a result, the eye sees the light pulses associated with these sub fields with double field frequency.
In yet another embodiment the scan driver is configured such that the address periods of the first electrodes coincide in time. This has the advantage that common circuitry can be used to address the whole PDP for every sub field, independent of the length of the display period of a sub field.
In an embodiment which has the same advantage, the timing circuit supplies an order of weight factors for the sub field periods of the two groups such that the weight factors of display periods of corresponding sub field periods differ minimally. Further, let us assume that the sub field order of two consecutive rows is different. This implies that after the common addressing period of a certain sub field in the field, the duration of the subsequent display period differs for the two rows. To be able to again have a common addressing period for the next sub field, an idle period occurs for the row with the shortest display period. This lost idle time is minimal if the duration of the display periods corresponding to a same addressing period differ minimally. This is the case if the weights associated with the corresponding display periods differ minimally.
In an embodiment of the invention when the received display information comprises data words having binary coded bits corresponding to weights, the timing generator generates weight factors of the display periods within a field period such that each weight factor corresponds to the weight factor for one of the bits, and the weight of the sub fields corresponds to the weight value for each of the bits of the data word, such that a minimal number of sub fields are required.
These and other aspects of the invention will be apparent from and elucidated with reference to the accompanying drawings.